1. Field of the Invention
Embodiments of the present invention generally relate to satellite navigation receivers. More particularly, embodiments of the present invention pertain to a configurable acquisition and tracking engine architecture for use in GPS receivers to provide enhanced functionalities and improved performance.
2. Description of the Related Art
Satellite navigation systems are made up of multiple, specially designed satellites orbiting Earth. These satellites continuously transmit precise, specific radio frequency (RF) signals that are used to provide navigation information. One widely adopted satellite navigation system is referred to as the Global Positioning System (GPS). By processing the RF signals from four or more GPS satellites, a GPS receiver can determine its current location (longitude, latitude, and altitude) fairly quickly and with a good degree of accuracy.
This remarkable feat is accomplished, in part, by assigning each satellite its own unique coarse acquisition (CA) code. The satellites continuously transmit its own CA code in a spread spectrum, code division multiple access (CDMA) format. When a GPS receiver is first turned on, it searches for signals that match any of the known CA codes that are made up of a complex pattern of specific 1023 pseudo-random bits. This search is typically performed by an acquisition engine. Thereby, a GPS receiver can identify which of the satellites transmitted the received signals by examining the CA code contained in the acquired signal. Furthermore, once a signal is acquired, the CA code phase provides timing information that is used to calculate the distance of the GPS receiver from the satellite. Distance measurements to three satellites can be used to accurately triangulate the GPS's current location. A fourth satellite measurement is used to resolve timing issues. Thereafter, a tracking engine is used to keep tracking the GPS signals so that the GPS receiver can continuously calculate and provide its location information.
Designing acquisition and tracking engines is quite a challenge. This is because the GPS satellites transmit the signals containing the CA codes at only 22 watts of power. The signals must travel over 12,000 miles through space and Earth's atmosphere. By the time the signals are received, they are extremely weak and attenuated. In fact, the power (−130 dBm) of these signals upon reception, are below the noise floor. Furthermore, there are many impediments to receiving signals from GPS satellites. One factor relates to the fact that GPS satellites orbit the earth approximately every twelve hours. Due to the relatively high velocity of the satellites, their signals will always exhibit some Doppler induced distortion. In addition, the transmitted signals operate on substantially a line-of-sight basis. Consequently, they are susceptible to being easily blocked or distorted by buildings and/or terrain. Moreover, line-of-sight signals may be prone to multipath distortion. The GPS band also falls within a crowded frequency spectrum and may be suffer interference issues with signals being transmitted on either side of the GPS band. Other interference sources that may detrimentally impact signal reception are thermal noise, atmospheric propagation interference, etc.
In order to overcome all of these potential impediments, the acquisition and tracking engines tend to be dedicated, highly specialized digital circuits. Their designs are tailored to acquire and track the CA codes as fast as possible and with as much precision as possible to meet certain stipulated cost, size, and power consumption constraints. For example, GPS receivers for use in cell phones are optimized for size and power consumption. In contrast, GPS receivers for survey equipment are optimized for accuracy. The end results are a wide host of rigid, static acquisition and tracking engine designs that have been optimized for the singular purpose of meeting certain one or more target goals in terms of either speed, accuracy, cost, size, and/or power consumption.
The downside to developing highly specialized acquisition and tracking engines is that GPS receivers may encounter different environments. The conditions for receiving the CA code signals are subject to change, sometimes drastically, in a short span of time. This is especially prevalent for mobile applications. Unfortunately, today's dedicated acquisition and tracking engines typically lack the flexibility to adapt to rapidly changing operating conditions and different environments.